Filter elements of the above type are known and used in the filtration of air, for example, in air conditioning apparatus. The two single filter elements form a bag and in most cases are fleece pads made of synthetic fibers. The flocks used in them as the raw material are carded and thus converted into a gauze. This gauze is then fed into panels and when it has reached the desired final weight, the panels are stapled.
For the filtration of air, a layer of glass fibers is used and this layer is covered on one side with a layer of gauze web or needle felt. Two such sandwiched, single filter elements are then placed on one another with their glass strata facing each other and stapled together on three sides along courses running parallel from the open side of the closed end.
Heretofore, it was customary to unite these two single filter elements by sewing or fusing them together at the indicated points. Additional threads still had to be drawn in between every two parallel courses, in order to restrict the size of the opening. In such filter elements there were consequently threads on every side which could not be tucked in and tied up. Furthermore, the production is comparatively expensive, since special sewing machines are required and, additionally, every seam still had to be individually cemented.